What if Thanos briefly shows up, takes an Infinity Gem that was on Earth, continued on his way and then Ultron is born from the need to have a more immediate response unit if the Avengers take too long to assemble?

Thor in Asgard, Hulk in the middle of nowhere. Even if Iron Man got to Thanos in a matter of minutes, he'd be taken down in humiliating fashion. So, he could talk to Pym about developing a collection of Iron Man-eque robots he could lead until the other Avengers arrive.

If Thanos shows up briefly in any film it's going to be in THOR II, because of his connection with Loki and the Infinity Gauntlet.